Radiotherapy has a long history in the treatment of malignant tumors of the body. Many malignant tumors are treated by radiotherapy, either alone or in some combination with surgical resection and/or systemic administration of antiproliferative agents. While radiation can successfully stop tumor growth, its side effects are well known. One serious side effect is scar formation adjacent to the site of radiation. Such scar formation often results in damage to or constriction of important structures located behind the target radiation site.
A similar situation exists with regard to intraocular tumors. The majority of adult malignant tumors of the eye are malignant melanoma. They are treated by enucleation if they are large, or by radiotherapy, or by surgical resection of the tumor.
In the eye, radiation damages not only the tissue surface, but often damages tissues that are adjacent to the target radiation site. These side effects produce cataract formation, radiation retinopathy, radiation optic nerve neuropathy, and vitreous hemorrhage. Radiation retinopathy manifests itself as a result of damage to the vascular supply of the retina and optic nerve. Neuropathy is induced by damage to the vascular supply of the optic nerve head or the optic nerve itself. The severity of the side effects often relate to the location of the tumor; for example, close to the fovea, the most sensitive structure of the retina, or close to the optic nerve head.
Two forms of radiation therapy are used to treat malignant tumors of the eye. One form is external beam radiation, where the source of radiation can be an x-ray or proton beam or helium ion radiation. The second form is the use of radioactive plaques or radioactive seeds which are sutured to the sclera behind the tumor.
In external beam radiation, the radiation passes through normal body structures before it reaches the target radiation site, which is the tumor. After reaching the tumor, it also penetrates tissues located adjacent to the tumor, such as tissues on the side of and behind the tumor. In radioactive plaque administration, such as with .sup.125 I or radioactive seeds, radiation is generally unidirectional and hence the beam scatter is minimized. The forward direction of the beam, however, will not limit itself to the tumor, but also penetrates normal structures of the eye. Therefore, in treating intraocular tumors structures such as the lens, retina, or optic nerve can also be damaged. A similar situation exists in other areas of the body, such as in the treatment of prostate cancer, lung tumors, or tumors located in the neck or base of the skull.